Propylene resin materials as various industrial parts are usually subjected to practical use as coated at least partly. However, because propylene resins have no polar group in their own structure, it is difficult to apply a coating directly thereon to obtain a sufficient coating adhesion for practical use. Improvement on adhesion of the molded article to a coating has been made in most cases by cleaning the surface of the molded article with trichloroethane (TCE) followed by primer coating or a plasma treatment.
However, these conventional coating methods have their several problems. That is, the use of TCE cleaning agent has been prohibited since 1995 by the Montreal protocol from consideration of global environmental conservation. Primer coating is costly because of expensiveness of a primer itself and also the increase in coating steps. Besides, the solvent in the primer must be evaporated, which involves a danger of fire.
The plasma treatment requires installment of expensive equipment for creating a high degree of vacuum and should be carried out in a batch system, which unavoidably increases the coating cost. Further, in recent years molded articles of complicated shape have been demanded. It is difficult to give a uniform plasma treatment to all the surfaces of an article having a complicated shape. Furthermore, if a plasma-treated surface is contacted with foreign matter, the adhesion of coating reduces, which may result in unevenness of coating performance. Therefore, considerable care should be taken in handling a plasma-treated article.
Omission of the cleaning treatment with TCE, primer coating and plasma treatment that have hitherto been required in coating propylene resin molded articles could lead to simplification of coating steps, improvement on the working environment, and reduction of cost. Many studies have so far been made in an attempt to omit these steps.
For example, a resin composition containing hydroxyl-modified polypropylene (JP-B-5-64660, the term "JP-B" as used herein means an "examined published Japanese patent application"), a resin composition containing unsaturated carboxylic acid-grafted polypropylene (JP-A-5-59231, the term "JP-A" as used herein means an "unexamined published Japanese patent application"), a resin composition containing a diene polymer, EPR, and maleic anhydride-modified polypropylene (JP-A-7-118488), and a resin composition containing a hydroxyl-containing hydrogenated copolymer and an elastomer (JP-A-7-149971) have been proposed to date.
However, when coated directly with a coating, molded articles of these resin compositions do not always have sufficient adhesion to the coating. In particular, where well-balanced high mechanical performance, especially high rigidity combined with high impact strength, is required of a molded article, either a TCE treatment or primer coating is virtually essential in order to obtain practically acceptable adhesive strength of coating.
Above all, when a melamine coating is used, it has been difficult to secure all of sufficient coating adhesion for practical use, satisfactory appearance, and well-balanced high mechanical strength (i.e., high rigidity and low-temperature impact strength).
The inventors of the present invention have previously proposed a propylene resin composition containing a filler, a specific elastomeric component, and a modified propylene polymer as disclosed in Japanese Patent Application No. Hei 6-178944 (corresponding to JP-A-8-041276). In this proposal, they have succeeded in achieving practically sufficient adhesion to coating and yet securing excellent physical properties in good balance with processability even when a melamine coating which generally exhibits poor adhesion is applied and baked at 140.degree. C., which is relatively lower than usually employed for baking, i.e., under a more strict condition.
There has been a tendency to reduce the thickness of a coating film provided on a molded article as well as the thickness of the molded article itself from the today's demand for cost reduction. To cope with this tendency, an improvement in adhesion to coating has been demanded. Because not a few molded articles are coated partially, an improvement in appearance (i.e., reduction of flow marks) of a molded article as molded or non-coated has also been demanded.